Self-sealing Pressurized Limb Enclosure

ABSTRACT

Method and system are provided for creating a self-sealing pressurized limb enclosure for the assessment of pressure effects on the limb. Embodiments can be self-sealing in that the seal is created by the positive pressure in the enclosure relative to the external environment and does not necessitate contact pressure at the seal location that exceeds the pressure in the enclosure. The seal accounts for anatomical size differences as well as deformations in the size and shape of the limb due to pressure. Furthermore, the seal maintains function in the presence of skin and tissue movement. In operation, the system can be used by an individual without external assistance.

TECHNICAL FIELD

The present invention relates to the field of methods and apparatusesfor managing the pressure around a limb.

BACKGROUND ART

In some medical applications it is desirable to study the effects ofpressure on a limb. One example is the optical determination of centralvenous pressure from the dorsal hand veins as described in U.S.provisional patent application 62/423,768, incorporated herein byreference. In such applications, it can be important that the methodused to generate pressure around the limb does not create additionalcontact pressures on the limb that exceed the pressure of interest. Thecreation of a pressurized enclosure around a limb in a manner that doesnot utilize contact pressures exceeding the enclosure pressure is achallenging problem. The difficulty is exacerbated by the physicalcomplexity and anatomical variability inherent to human limbs, as wellas by the desire that the sealing mechanism be easily used by a singleoperator.

SUMMARY OF INVENTION

Embodiments of the present invention enable the creation of aself-sealing pressurized limb enclosure for assessment of pressureeffects on the limb by successfully addressing many nuances associatedwith human physiology and anatomy. Embodiments address criteriaassociated with the intended use by providing a system where the contactpressure at the seal location does not exceed the enclosure pressure orcreate significant local pressure gradients along the limb. Due to thephysiological properties of the limb, the seal mechanism should functionin the presence of skin and tissue deformations as well as movement ofthe tissue relative to the enclosure boundary.

Embodiments also provide other advantages associated with usability andcomfort. Embodiments function in a manner that allows an individual tooperate the system without additional assistance. Embodiments facilitateuser comfort by not requiring the user to resist the forces acting ontheir limb due to the positive enclosure pressure.

An example seal mechanism comprises a rigid outer aperture and an innerflexible seal. The rigid outer aperture couples with the rigid enclosureand allows entrance of the hand into the enclosure. The aperture sizecan be adjusted to accommodate various sizes and shapes of the limbsunder examination. The inner flexible seal compresses radially on to thelimb due to the positive pressure in the enclosure, and is thereforeself-sealing. The flexible seal accommodates deformation of the softtissues and subtle movements of the limb within the aperture. The systemmaintains seal integrity in the presence of skin movement relative tothe underlying bone structure.

Embodiments provide physical and geometrical properties of the innerseal that are important to creating an effective air seal. The seal issufficiently compressible in the radial dimension to uniformly andconsistently restrict airflow. At the same time, the seal resists forcesin the axial dimension; in some embodiments this is achieved viafriction with the limb, axial rigidity, or other means of stiffness orresistance to deflection in the axial dimension. The circumference ofthe inner seal is equally important: the inner seal must also allowentrance of the terminal aspect of the limb (i.e., the hand or foot),which may have a larger diameter than more proximal aspects of the limb,and in general is constructed such that it does not not generate anycircumferential pressure on the limb that exceeds the enclosurepressure.

The distance or gap between the rigid outer aperture and the surface ofthe limb is an important parameter. A large gap increases the axialforces acting on the seal and the limb; excessive force will result inuser discomfort and potentially eject the inner seal and limb from theenclosure. A smaller gap reduces the axial force such that an air sealcan be maintained. Embodiments offset these axial forces via limbsupport mechanisms so that the user does not have to activate muscles orotherwise resist limb movement. Embodiments' use of an elbow stop oralignment of the limb such that movement is opposed by gravity, areexamples of solutions to mitigate the axial force.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a typical seal mechanism with the sealpressure exceeding enclosure pressure.

FIG. 2 illustrates directions of forces acting on the limb.

FIG. 3 is an example of the seal system under no positive pressure.

FIG. 4 is an example of the seal system under positive pressure.

FIG. 5 shows the forces present at the distal sleeve.

FIG. 6 is a force diagram depicting the conditions at the point ofcontact.

FIG. 7 illustrates the relationship between gap size and non-rigidaperture surface area.

FIG. 8 is an illustration of seal system using contact sensors.

FIG. 9 is an illustration of seal system using pressure sensors.

FIG. 10 is an illustration showing the forces acting on the limb.

FIG. 11 is an illustration depicting the change in seal location due toincreasing pressure.

FIG. 12 is an illustration of fold radius differences.

FIG. 13 is an illustration showing a fundamental concept of acompression seal.

FIG. 14 is an example of an axial-rigidity based seal.

FIG. 15 is a second example of an axial-rigidity based seal.

FIG. 16 is a third example of an axial-rigidity based seal.

FIG. 17 is a fourth example of an axial-rigidity based seal.

FIG. 18 is an illustration of multiple fixed apertures.

FIG. 19 is an illustration of a variable aperture using an irisdiaphragm.

FIG. 20 is an illustration of a variable aperture using overlayingleaves.

FIG. 21 shows the influence of aperture size and seal material on sealeffectiveness.

FIG. 22 shows the influence of aperture size and seal material on sealmovement.

FIG. 23 is an illustration of a variable aperture using overlayingbristles.

FIG. 24 is an illustration of angular relationships concerning materialfolding in some embodiments.

DESCRIPTION OF EMBODIMENTS AND INDUSTRIAL APPLICABILITY

Definitions

Seal Junction describes the area over which there is contact between theflexible sleeve and the tissue of the limb.

Seal Location is the location of the seal junction relative to definablelocation, such as the plane defined by the rigid aperture. RadialPressure is the pressure normal to the limb surface acting towards thecenter of the limb.

Axial Force is the pressure acting along the axis of the limb. Apositive axial force acts to push the limb out of the enclosure.

Pressure Tolerance defines the permissible limit of variation inpressure relative to a set or desired value. The pressure tolerance fortypical applications is roughly 1 cm H₂O.

Pressure Consistency defines a static condition where the pressureacross a surface is consistent to within the pressure tolerance, i.e.,local pressure gradients larger that the pressure tolerance are notpresent.

Non-positive angular progression configuration: as used in this documentdefines a configuration where progression around the circumference ofthe seal material results in a condition where the angular relationshipbetween sequential point on the circumference does not result in anincrease of the angle define by an line from the center of the objectand the intersection with the material forming the seal. As illustratedin FIG. 24A, a circle maintains a positive and constant angle ofprogression. As illustrated in FIG. 24B, as the seal material begins tofold on itself, the angle of progression can decrease and become lesspositive as the material begins to form a fold. As illustrated in FIG.24C, further formation of the seal creates a situation where the angleof progression become zero or can be negative as the material begins tofold back on itself. From another perspective, in a non-positive angularprogression configuration a line drawn from the center of the objectoutward encounters the surface of the seal material more than once.

Tube: as used in this document simply defines a cylindrical object fortransporting with a proximal and distal opening. The object can vary incircumference along the length of the tube.

Sealing engagement, or sealingly engaged, or seal, refers to anengagement between two entities, such as between a sleeve and a limb,that provides adequate resistance to airflow. Sealing engagement doesnot require absolute airtightness or zero air flow through theengagement, but only sufficient restriction to air flow that theengagement facilitates the desired pressure differential across theengagement.

Properties and Features of Example Embodiments

For the intended use of studying the effects of pressure variations on alimb, the following system capabilities are provided by various exampleembodiments:

The pressure at the seal junction should not exceed the pressure of theenclosure by more than the pressure tolerance. FIG. 1 shows a typicalapproach for creating an air seal. The pressure at the seal locationexceeds the pressure in the enclosure thus creating an effectiveresistance to air flow out of the enclosure. The use of such a standardseal design creates a local area of increased pressure that acts as atourniquet and influences measurable pressure effects in the distallimb. Such localized pressure does not satisfy requirements forapplications that can be accommodated by embodiments of the presentinvention.

The seal junction creates pressure consistency around the circumferenceof the limb. Spatial variances in the seal quality can create failurepoints that allow air to escape via high velocity flow. Air leakagecreates localized pressure gradients and areas of skin deformation,permitting further air leakage. A seal with pressure inconsistencyaround the limb is unstable and unreliable, and unsuitable for theintended uses.

The seal system can compensate for large anatomical variations in thesize and shapes of limbs. This includes both variances betweenindividuals in a population, as well as the variance in the geometry ofthe limb within an individual. The typical limb increases in diameter asone moves proximally toward the point of attachment, though the diameterof the terminal limb element (i.e., hand or foot) can often exceed thelimb diameter at more proximal locations. The seal mechanism canaccommodate varying limb diameter and maintain functionality if the seallocation moves along the limb.

The system can allow for some variance in the placement of the limbwithin the seal mechanism. It is anticipated that individuals will movetheir limbs slightly within the seal mechanism during any measurementprotocol. Embodiments of the present invention will tolerate or adapt tothese expected small variances in limb position.

Because the limb is a non-rigid object that deforms under forces, theseal can accommodate for changes in the size and shape of the limb.Limbs are complex, non-uniform objects composed of multiple tissuelayers including bone, muscle, fat, vasculature and skin. The differenttissue layers vary in their physical properties and some are easilydeformable. Specifically, the skin has a moderate degree of elasticityand can be compressed or stretched relative to the bones of the limb. Inaddition, the volume of vascular tissues is highly affected bysurrounding pressures. Embodiments can accommodate for changes in thesize and shape of the limb which that occur in response to variations inthe enclosure pressure.

Positive enclosure pressure relative to external environment will act topush the limb out of the enclosure, potentially creating anuncomfortable experience for the user. FIG. 2 shows key forces acting onthe limb. Radial forces are defined as those forces acting into the limbin a manner normal to the surface of the limb, while axial forces actalong the longitudinal axis of the limb. Embodiments provide that theaxial force experienced by the user is minimized or mitigated to theextent possible. The axial force out of the enclosure is defined by thecross-sectional area of the rigid aperture, which includes the limb andthe gap around the limb. Embodiments can manage the total axial force sothat the force pushing the limb out of the enclosure is tolerable anddoes not require the user to actively resist this force. Someembodiments include limb support mechanisms or other considerations thatact to oppose the axial force out of the enclosure and increase subjectcomfort.

To facilitate overall usability, embodiments can be operable by a singleindividual without assistance from another party. Specifically, the useris able to insert a limb into the device such that effective seal isformed without the assistance of a second individual. In someembodiments, the user can simply place their limb through the aperture.Many other user-friendly scenarios exist, but the general goal is tominimize the number of actions that must be performed by the user.

Embodiments of the present invention provide the advantages describedabove, and are effectively self-sealing because the pressure used tocreate the seal is generated by the pressure difference between thepositive pressure in the closure relative to the external environment.

System Components

Embodiments of the present invention involve the integration of threecomponents working in concert. Components include (1) an outer rigidaperture with variable opening that allows entrance of the limb into theenclosure; (2) an inner radially flexible material that is compressedradially to create an effective air seal; (3) a design element thatenables the seal to oppose the axial forces of positive pressure. Theproperties of each component and their integrative function aredescribed below.

The outer aperture is sufficiently rigid such that it is not deformed bythe enclosure pressure. A variable opening size is provided in someembodiments to accommodate limbs of different sizes. The variableaperture can take many forms. For example, the system can use acontinuously variable aperture, such as an iris diaphragm. Such anaperture can be opened to easily allow the limb entrance into theenclosure, and then can be closed to reduce the gap between the apertureand the limb. Alternatively, the system can employ a set ofinterchangeable fixed apertures that are sized to be as small aspossible while avoiding contact with the limb and allowing entrance ofthe limb into the enclosure.

An inner flexible material forms the air seal around the limb. The sealis created using the radial forces generated by the pressure in theenclosure, and in this way, is self-sealing. The radial force places thematerial used to create the seal under compression. Compression is aterm associated with the general forces on an object and used with anawareness that any bend of a material creates both tension andcompression. As used to describe the formation of the seal at around thelimb, the seal material is compressed around the arm to create a seal.The material properties of the seal are an important element of theinvention, and the seal must have sufficient radial flexibility suchthat it can be compressed to create pressure consistency.

The examples depicted herein generally show the limb extending past theend of a sleeve, for example having a tube encircle an arm while thehand extends past the end of the tube. The invention also contemplatessleeves with closed ends, for example a portion encircling an arm with aglove-like or mitten-like portion that also covers the hand. In exampleembodiments, an optical measurement is made of a limb while at least aportion of the limb is surrounded by a sleeve. The portion beingmeasured can be outside of the sleeve, or can be covered by, or evencompletely enclosed in, the sleeve, provided that the portion beingmeasured is accessibly to the measurement system. As an example, anoptically transparent glove end to an opaque tube can be suitable insome example embodiments. As used herein, the term sleeve contemplatesboth structures with ends through which a portion of the limb protrudes,and structures that similarly surround a limb while also enclosing theend of the limb while still providing access as required for themeasurement, e.g., an optically transparent portion.

The system also includes design elements that confer axial resistance orrigidity, enabling the seal to oppose the axial force of positiveenclosure pressure. Opposing forces can be generated by the material,geometrical, or structural properties of the seal. Examples of opposingforces include, but are not limited to, friction generated between theseal and the limb, stiffness associated with tension of the seal,stiffness associated with compression of the seal, and any combinationof the above.

System Operation

The constraint that the pressure on the limb not exceed the pressure inthe enclosure is satisfied by using a flexible seal whose primarymechanism for creating pressure on the arm is the result of the pressuredifference between the interior of the enclosure and the outside of theenclosure. FIG. 3 shows an example embodiment of this element. In thisexample, the flexible seal material is a sleeve that is attached to theinner surface of the enclosure, and axial resistance is provided byfriction between the sleeve and the limb. The aperture is circular inshape and the limb is modeled as a truncated cone. Additionally, thelimb is assumed to be centered in the aperture for ease of description.The figure shows several elements that define an effective seal systemfor the limb. An external rigid aperture, 101, defines the entrance intothe enclosure, 103. A flexible sleeve, 102, is attached to the enclosurein an airtight manner. The diameter of the aperture is denoted asdiameter a. The unilateral gap between the limb and the rigid apertureis defined as distance g. The limb diameter varies in the axialdirection, as is typical in most individuals. The distal diameter of thesleeve, 104, is larger than the largest diameter of the limb at the sealjunction, defined as diameter d. As pictured in FIG. 3, there is nopositive pressure in the enclosure and the sleeve is not compressedagainst the limb.

As the pressure in the enclosure is increased the seal system mustrespond in a manner that allows a positive enclosure pressure to becreated. FIG. 4 shows the seal system under conditions where theenclosure pressure is greater than the atmospheric pressure, and a sealaround the limb has been created. Under pressure, the flexible sleeve isunder compression in the radial direction and under tension in the axialdirection. The sleeve contacts the limb over an area of skin, 401, andis attached to the enclosure along area 402. The pressure differenceexerts force on the sleeve creating axial tension in sleeve, 403. At theseal junction, 401, the sleeve is forced into contact with the limb viaradial forces and has compressed, collapsed or folded under the pressuregradient to create an effective seal around the limb. The radialpressure collapsing the flexible sleeve places the sleeve undercompressive forces. The resulting air seal is a consequence of thepressure difference between the inside of the enclosure and the outsideof the enclosure.

The requirement that the pressure at the seal junction not exceed thepressure in the enclosure by a pressure tolerance necessitatesexamination of the distal aspect of the sleeve. FIG. 5 is anillustration of the forces present at the distal junction of the sleevewith the limb. The distal sleeve at the seal junction is subject tothree possible forces that must be managed appropriately. The majoractive force is radial compression of the sleeve against the arm causedby the enclosure pressure. A second possible force is the physicalweight of the sleeve pushing on the arm. The third possible force is acircumferential force or hoop force. To minimize the difference betweenthe pressure on the arm under the sleeve, 502, and the pressure on thearm in the enclosure, 503, to within the pressure tolerance, thematerial selected for the sleeve can be of minimal weight. As it relatesto minimization of circumferential force, the distal diameter of thesleeve is large enough that the distal aspect of the sleeve is not undertension and therefore does not generate circumferential forces. Sleevedesign based upon defined geometric considerations and the selection oflightweight material create a system that satisfies pressure criteria.

To create a functional seal, the forces acting on the sleeve functionmust sum to create a static condition. Otherwise, the seal would fail.FIG. 6 is a force diagram depicting the forces present at the area ofcontact between the sleeve and the arm. As illustrated, the sleeve issubject to an axial force pushing out of the enclosure, 702. Understatic conditions, an equal and opposite force is generated due to thefriction between the limb and the sleeve. The frictional force is theproduct of the pressure in the enclosure, the area of contact with thelimb, and the static coefficient of friction. The flexible sleeve musttherefore have sufficient length and the material must have a staticcoefficient of friction such that the static force of frictionsufficiently opposes the sleeve force.

A concurrent consideration is associated with minimizing the sleeveforce. The force on the sleeve is a function of the gap, g, between theaperture and the limb, as shown in FIG. 7. The force on the sleeve isthe product of the gap area and the pressure in the enclosure, and thesleeve force is minimized by minimizing the gap size. Preferably, therigid aperture is as close to the skin as possible, while ensuring thatdirect contact is avoided and that there is sufficient space for smallmovements of the limb.

Under preferable conditions, the limb does not contact the rigidaperture since such contact can create pressures that exceed thepressure tolerance. Contact sensors can be used to ensure that nocontact with the rigid aperture. FIG. 8 is an illustration of how suchcontact sensors, 801, can be used to determine the presence of contactbetween the limb and the hard aperture.

Pressure sensors can also provide valuable information to determinewhether the contact pressure is negligible. For example, when testingindividuals with less elastic skin, the gravitational pull on the tissuecreates a significant sag in the skin, resulting in contact with therigid aperture. The contact pressure due to sagging skin is often smalland beneath the pressure tolerance. Thus, the use of pressure sensors inthe aperture can distinguish between cases when contact pressure isnegligible and when it can interfere with the measurement and must beaddressed, e.g., by increasing the gap size. FIG. 9 shows an array ofpressure sensors, 901, concentrated on the bottom of the rigid aperturethat enable such a determination.

Understanding of the system also requires evaluation of the forcesacting to push the limb out of the enclosure. FIG. 10 shows that theaxial force acting to push the limb out of the enclosure is dependent onthe cross-sectional of the aperture, defined by diameter a, and thepressure in the enclosure. Opposing forces on the limb can includestatic friction between the limb and supporting elements. For example, aforearm enclosure can use a palm rest, 1001. Static friction between thehand and the palm can offset the axial force due to pressure. An elbowrest, 1003, can also be used as supporting element that creates staticfriction with the limb. If the axial pressure force exceeds thecumulative frictional forces, an elbow stop, 1002, can be added to thesystem. An elbow stop will oppose the movement of the forearm out of theenclosure and increase subject comfort because the subject will not feelthe need to actively resist the axial forces exerted on the limb. Also,the limb and enclosure can be oriented such that the axial pressure isdirectly opposed by gravity.

The use of a flexible sleeve creates a system that allows the seal tomove in the axial direction as the pressures on the skin create stretchof the skin. As the pressure in the enclosure increases, the sleeveforce will increase and stretch the skin in the axial direction. FIG. 11illustrates that the seal junction can move from location 1101 at lowpressures to location 1102 at higher pressures due to skin deformationwhile the bones and other more rigid structure remains nominally stablein position. Skin stretch is often modeled as a spring damper system asillustrated. The flexible sleeve seal system maintains operationalintegrity as the seal location moves due to both tissue movement andskin stretch.

When using a flexible sleeve as the mechanism to create a seal, theformation of an effective seal around the limb is dependent uponmaterial selection with attention given to the fold radius. The foldradius is the radius or curvature defined by the material under definedpressures. For visualization purposes, consider a very thin pliablepiece of plastic folding back on itself. The material effectively foldsback, and the resulting fold radius is remarkably small. In contrast, apiece of carpet when folded back on itself has a significant foldradius. The fold radius is defined by the physical and geometricproperties of the material.

FIG. 12 is an illustration communicating the importance of fold radius.As shown, there are two flexible sleeves surrounding the upper half of alimb. Both are subjected to the same pressure, but the responses of thesleeves are dramatically different. The material on the right, 1201, haseffectively folded upon itself utilizing a very small fold radius toeffectively create an air seal. In contrast, the sleeve used on theleft, 1202, has a much larger fold radius and may fail to create aneffective air seal. If the bend radius of the sleeve is large atpressures used in the enclosure, then seal quality will be compromisedand the uniformity of the seal across circumference of the limb will bepoor. In general, if the material used for the seal cannot effectivelyfold onto itself with a small fold radius, the overall seal quality iscompromised resulting in an unstable and unreliable seal. In contrast,if the material has a suitably small fold radius and can effectivelyfold back on itself, a stable and reliable seal will be created.

A primary material property affecting fold radius is the elasticmodulus; the geometrical properties of the material, primarilythickness, are also important. A flexible sleeve can be selected suchthat the thickness and elastic modulus properties enable a small foldradius and create an air seal at the enclosure pressures. Materials thatcan satisfy these criteria include, but are not limited to, elasticmaterials such as latex or silicone, moderately inelastic material suchas high-density polyethylene or low-density polyethylene, and fabricmaterial such as nylon, Kevlar, and terylene. The above list is notconsidered an exhaustive list of materials that may satisfy the flexiblesleeve criteria but rather a list of example materials.

The fact that the terminal limb diameter is often larger than the moreproximal limb diameter in most individuals makes it desirable, but notnecessary, to use a sleeve element with elastic properties. In thiscase, the sleeve stretches over the larger diameter appendage and formsa distal circumference more consistent with the size of the limb.Elastic material properties are also desirable because they allow asleeve to return its original size and position when the deformingforces are removed. Inelastic or viscoelastic materials may not returnto their original size and shape without the application of otherforces, or may return slowly, limiting the temporal response of thesystem.

The example embodiments satisfy all the criteria described. The use ofradial compressive forces to create a seal around the limb meets therequirement that the seal pressure does not exceed the enclosurepressure. The concurrent use of the flexible sleeve with sufficientfriction with the limb and a minimal gap between the limb and the rigidaperture creates an overall seal system that is effective and easy touse. In use, the user simply places their limb into the enclosurethrough the flexible seal. As the pressure increases in the enclosure,the flexible sleeve creates a self-sealing closure around the limb, andthe axial pressure force on the seal and the limb is opposed by frictionand other design elements.

Additional Embodiments

Axial Rigidity-Based Seal System

The embodiments described above used the example of a seal system wherethe opposing force to the axial pressure was provided by frictionbetween the seal and the limb. The present invention also provides aseal system based upon axial rigidity of the seal. These exampleembodiments are not based upon a consideration that the forces due tostatic friction oppose the air pressure; rather, the seal provides axialcompressive strength that opposes the air pressure. FIG. 13 showsimportant elements of the concept. As the pressure in the enclosureincreases, the air pressure force is opposed by the structural elementsof the seal mechanism. The structural elements can be solid, can deformunder pressure, or can act like a spring. As shown in FIG. 13, smallpressure forces result in a smaller degree of compression whereasincreased pressure forces can create further compression. The stiffnessor rigidity of the seal elements resist this compression. As illustratedin the figure, there is not a requirement of static coefficient offriction to oppose the axial pressure force and at the extreme, intheory, the system can operate effectively with a frictionless surface.

The concepts demonstrated in FIG. 13 can be used to implement a varietyof seal mechanisms. FIG. 14 is an example of a seal mechanism based onresistance to axial compression. The seal mechanism has an axialrigidity that is used to oppose the force of air pressure. The sleeve iscomposed of a flexible sleeve with embedded battens, 1401. Battens areused in sails to add additional rigidity to the sail in a desireddirection. For the seal system, the battens are composed of alightweight material that confer axial rigidity. As the pressure in theenclosure increases, the axial pressure force will largely place thesleeve element into compression, rather than tension. The compressivestrength of the battens resists deformation due to the axial pressureforce while maintaining the radial flexibility of the sleeve such thatthe distal aspects of the sleeve can conform to the limb and create aneffective seal. The system does not have requirements regarding thestatic coefficient of friction between the sleeve and the limb, thoughin practice, some static frictional force will be present and willadditively combine with the axial sleeve rigidity to oppose the axialpressure force.

The resulting seal system satisfies the design requirements butaccomplishes these goals without creating significant axial stress atthe skin surface. Depending upon application nuances, the reduction ofskin stress might be a desirable attribute. The reduction of skin stresscan be important in older individuals that have more fragile skin.Additionally, the degree of skin stress can be influenced by materialselection and specifically by use of materials that have a minimalcoefficient of friction of the material including the distal sleevelocation.

A second embodiment of an axial rigidity-based seal system is shown inFIG. 15. As shown, the thickness of the seal element, 1501, varies alongthe axial dimension, with greatest stiffness and rigidity at the pointof attachment to the enclosure. The distal seal is designed to retainsufficient radial flexibility to create an effect seal, and the axialrigidity conferred by the increasing thickness opposes the axialpressure force on the seal. In addition to or in alternative to changingmaterial thickness in the axial dimension, the material of the sealelement can also be varied along this axis to increase stiffness.Axially varying the material properties can be achieved by “doping” theseal element with stiffness enhancing agents, or inter-weaving fibers orfilaments with axial rigidity.

A third embodiment of an axial rigidity-based seal system is shown inFIG. 16. The distal seal, 1601, is composed of a radially flexiblematerial to enable adequate seal formation between the sleeve and arm.In the axial direction, the seal designed somewhat like an accordionwith material characteristics that oppose the axial force of airpressure out of the enclosure. Elements of the system may be placed incompression or tension when acted upon by the axial pressure force. Dueto the accordion nature of the structure, each curve represents asituation of compression on the inner radius and tension along the outerradius. The mechanical rigidity of the bellows acts to offset the axialpressure from the enclosure. It is important to note that the bellowmechanism obtains additional rigidity at the point the bellows contacteach other. Specifically, at the point the bellows are collapsed on eachother, they generate a static coefficient of friction between adjacentbellows, which results in additional structural rigidity. At location1602, the physical height of the bellows increases under compression andcan obtain a height such that it becomes exceedingly difficult for theseal mechanism to be forced through the gap. Thus, this seal design maybe less influenced by the gap size then prior systems. As noted above,as the system compresses on itself, the bellows structure becomesincreasingly rigid. As this occurs the effective gap size becomesextremely small since as a rigid structure files the gap area. Such asystem may have benefits in terms of reducing the necessity for variableapertures.

FIG. 17 shows a fourth example of an axial rigidity-based seal system.This system does not utilize a continuous flexible sleeve, but instead aplurality of overlapping lightweight leaves. The leaves, 1701, are rigidin the axial direction and are designed to overlap to create aneffective air seal. The leaves are able to bend and flex at the point ofattachment, 1702, thus enabling a radially flexible seal at the distalseal element. In implementation, the large surface area of the leaf,1703, can create a location of high pressure as the leaf flexes from thesolid aperture location. This pressure point issue can be mitigated byusing a sheath that displaces the force over a wider area, 1704, to meetthe requirements of pressure tolerance. A similar embodiment shown inFIG. 21 uses overlapping filaments or bristles rather than leaves tocreate the leaves. Bristles offer axial rigidity with radialflexibility, and with sufficient overlapping can create an effectiveseal over the surface of the limb.

Variable-Sized Apertures

A variable aperture system can be implemented by using a set of fixedapertures that vary in size. FIG. 18 shows an example of such a system.A rigid disk, 1801, forming the aperture is attached to the front panelof the enclosure, 1804. The disk can be easily attached and removedusing quick-release elements, 1802, that allow optimization of theaperture size. A flexible sleeve to form the seal is attached to the lipof the disk at 1803, not shown.

Variable Iris with Flexible Sleeve

A continuously variable aperture system is illustrated in FIG. 19. Thesystem uses an iris diaphragm to allow convenient adjusting of theaperture size. The user can open the aperture wide using adjustmentlever 1901 to allow entrance of the limb into the enclosure, then reducethe aperture to minimize the gaps size around the more proximal limb.The individual leaves of the iris can be coated with a rubberized paintto ensure that the surface created by the leaves resists air flow.

FIG. 20 shows a second example of a continuously variable aperturesystem. The system design and operation have a similar configuration asin a common vegetable steamer, where overlapping leaves can create avariable aperture. A rigid cylinder, 2002, is threaded into the frontplate of the enclosure. The limb passes through the cylinder and intothe enclosure. Turning the cylinder forces the leaves open, creating aneasy-to-use adjustable aperture. Similar to the iris diaphragm, theindividual leaves iris can be coated with a rubberized paint to ensurethat the surface created by the leaves resists air flow. Alternatively,the sealing element, for example, a flexible sleeve, can be fitted overthe outer surface of the leaves to prevent air flow.

Demonstration of Applications

We include experimental data to demonstrate the principles outlinedabove. Data were collected from a single subject using an enclosurearound the forearm. Aperture sizes were varied using a set of rigiddisks, as described and shown in FIG. 18. A flexible sleeve was used tocreate a seal. The material used for the sleeve was varied todemonstrate the importance of physical and geometrical properties.Utilized sleeve materials included thin silicone (less than 0.5 mmthick, in the example 0.42 mm thick), thick silicone (between 0.5 mm and3 mm thick, in the example 1.05 mm thick), and no sleeve at all. Thethin and thick silicone sleeves had similar static coefficients offriction on the skin. The subject's forearm position was adjusted suchthe gap size between the arm and the rigid aperture was effectively zerofor the smallest aperture diameter of 2.75 in. The gap size thenincreased linearly with aperture diameter. Each experiment was repeatedfour times to assess variability.

FIG. 21 shows the maximal pressure attainable in the enclosure usingdifferent sleeve materials and different apertures. Due to residual airleaks in the enclosure, the maximal possible pressure attainable when noair is flowing around the arm was 47.5 cm H2O. The thin silicone sleeveachieved near maximal pressure regardless of the aperture size due to(1) a small fold radius that allows an effective seal to be created and(2) a suitable static coefficient of friction. In contrast, the thicksilicone sleeve created a less effective and highly variable seal due tothe larger fold radius, which allowed air leaks.

FIG. 21 also demonstrates the advantage of a flexible seal due todeformation of the arm. When the enclosure pressure was equal toatmospheric pressure, there was effectively no gap between the arm andthe rigid aperture. However, as positive enclosure pressure wasgenerated, the skin and other tissues deformed, allowing for significantair leaks that precluded formation of an effective seal.

FIG. 22 shows the influence of gap size on the forces acting on thesleeve. In each experiment, the enclosure pressure was increased to aset value of 35 cm H2O and the axial movement of the sleeve relative toits starting position was recorded. Static conditions are achieved whenthe friction with the arm and tensile forces in the sleeve oppose theaxial pressure force. In agreement with the equations in FIG. 7, theforce acting on the sleeve increases with the aperture diameter andhence gap size. Although not observed in these experiments, if thesleeve length is too short, the coefficient of friction too low, or theenclosure pressure too high, the sleeve can be forced out of theenclosure to constitute total seal failure.

The present invention has been described in connection with variousexample embodiments. It will be understood that the above description ismerely illustrative of the applications of the principles of the presentinvention, the scope of which is to be determined by the claims viewedin light of the specification. Other variants and modifications of theinvention will be apparent to those skilled in the art.

1. (canceled)
 2. A sealing apparatus as in claim 27, wherein the sleevecomprises a material that has a non-positive angular progression atpressures relative to ambient of 30 cm H2O or more.
 3. A sealingapparatus as in claim 27, wherein the sleeve comprises a flexiblematerial that comprises one or more of latex or silicone, high-densitypolyethylene, low-density polyethylene, nylon fabric, Kevlar fabric, andterylene fabric.
 4. A sealing apparatus as in claim 27, furthercomprising a plurality of contact sensors disposed relative to thesleeve and the pressurizable enclosure such that they sense contactbetween the limb and a rigid portion of the sealing apparatus or thepressurizable enclosure.
 5. A sealing apparatus as in claim 27, furthercomprising a plurality of pressure sensors disposed relative to thesleeve and the pressurizable enclosure such that they sense pressureexerted on the limb by a rigid portion of the sealing apparatus or thepressurizable enclosure.
 6. A sealing apparatus as in claim 27, whereinthe sleeve further comprises a plurality of battens, each batten beingstiff in the axial direction, mounted with the sleeve such that thebattens resist deformation of the sleeve out of the pressurizableenclosure.
 7. A sealing apparatus as in claim 27, wherein the sleeve hasan axial rigidity that is greater near the engagement with the openingthan distal from the engagement with the opening.
 8. A sealing apparatusas in claim 7, wherein the sleeve has an axial rigidity that smoothlydecreases from the engagement with the opening to a region distal fromthe engagement with the opening.
 9. (canceled)
 10. A sealing apparatusas in claim 27, wherein the sleeve is configured with accordion foldsand has resistance to folding such that pressure above ambient in thevolume compresses the accordion folds.
 11. A sealing apparatus as inclaim 10, wherein the sleeve material has a low coefficient of frictionwith the surface of the limb.
 12. A sealing apparatus as in claim 10,wherein the sleeve accordion folds compress at a lower pressure than thesleeve compresses to sealingly engage the limb.
 13. A sealing apparatusas in claim 27, wherein the sleeve comprises a flexible material thatcomprises thin silicone, thick silicone, or a combination thereof. 14.(canceled)
 15. (canceled)
 16. (canceled)
 17. (canceled)
 18. (canceled)19. (canceled)
 20. (canceled)
 21. (canceled)
 22. A limb seal apparatusas in claim 27, wherein the sleeve is configured such that the limbextends past the distal end of the sleeve into the enclosure. 23.(canceled)
 24. The limb seal apparatus of claim 27 where the sealchanges its physical configuration to obtain a non-positive angularprogression configuration at pressures less than 30 cm H20.
 25. The limbseal apparatus of claim 27 where the sleeve does not exert pressureabove a predetermined threshold when the enclosure is pressurized aboveambient.
 26. The limb seal apparatus of claim 27 wherein the flexiblesleeve is attached to the enclosure and is subject to increasing axialtension due to increasing pressure.
 27. A limb seal apparatus for usewith a pressurizable enclosure, comprising a flexible sleeve that allowsa limb to pass through the sleeve where the sleeve changes its physicalconfiguration to create an air flow restriction responsive to a pressuregradient between the inside and outside of the enclosure, wherein theair flow restriction is sufficient to result in transmural pressure ofzero in the veins of a limb in the enclosure.
 28. The sealing apparatusof claim 27 wherein the sleeve has a proximal attachment to an apertureallowing access to the interior of the enclosure, and a distal aperturethat circumferentially encloses the limb, wherein the sleeve's change inphysical configuration are characterized by pressure increases in theenclosure causing the portions of the distal sleeve to compress againstthe limb while elements of the sleeve in proximity of the apertureexperience axial tension.
 29. The limb seal apparatus of claim 27,wherein the sleeve comprises an air resistant material with asymmetricmaterial properties, in a configuration such that the material'sresistance to compression is greater aligned with the axis of the limbthan orthogonal to the axis of the limb.
 30. A limb seal apparatus foruse with a pressurizable enclosure, comprising a flexible sleeve thatallows a limb to pass through the sleeve where the sleeve changes itsphysical configuration to create an air flow restriction responsive to apressure gradient between the inside and outside of the enclosure,wherein sleeve mounts with the enclosure with a gap between an openingin the enclosure and the limb, and wherein the force due to frictionbetween the seal and the limb when the enclosure is pressurized plus thesleeve's resistance to axial deformation is at least equal to the forceon the seal due to pressure on the gap at pressures above a firstpredetermined threshold.
 31. The limb seal apparatus of claim 30,wherein the force due to friction between the seal and the limb when theenclosure is pressurized plus the sleeve's resistance to axialdeformation is less than the force on the seal due to pressure on thegap at pressures below a second predetermined threshold.
 32. (canceled)